Method of transmitting data in a communication system

ABSTRACT

A method of encoding a signal to be transmitted from a terminal via a channel in a communication network includes receiving the signal at the terminal; determining characteristics of the channel; encoding a first portion of the signal in accordance with a first encoding method to produce a first encoded signal portion; and encoding a second portion of the signal in accordance with a second encoding method to produce a second encoded signal portion. The first portion of the signal encoded in accordance with the first encoding method is dependent on the determined characteristics of the channel. The first encoded signal portion and the second encoded signal portion are transmitted via the channel.

RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 or 365 to GreatBritain, Application No. 0705328.3, filed Mar. 20, 2007. The entireteachings of the above application are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to communication systems. Moreparticularly the present invention relates to a method and apparatus forencoding, transmitting and receiving data in a communication system.

BACKGROUND

In a communication system a communication network is provided, which canlink together two communication terminals so that the terminals can sendinformation to each other in a call or other communication event.Information may include speech, text, images or video.

Modern communication systems are based on the transmission of digitalsignals. Analogue information such as speech is input into an analogueto digital converter at the transmitter of one terminal and convertedinto a digital signal. The digital signal is then encoded and placed indata packets for transmission over a channel to the receiver of anotherterminal.

Each data packet includes a header portion and a payload portion. Theheader portion of the data packet contains data for transmitting andprocessing the data packet. This information may include anidentification number and source address that uniquely identifies thepacket, a header checksum used to detect processing errors and thedestination address. The payload portion of the data packet includesinformation from the digital signal intended for transmission. Thisinformation may be included in the payload as encoded frames such asvoice frames, wherein each frame represents a portion of the analoguesignal.

Degradations in the channel on which the information is sent will affectthe information received at the receiver. Degradations in the channelcan cause changes in the packet sequence, delay the arrival of somepackets at the receiver and cause the loss of other packets. Thedegradations may be caused by channel imperfections, noise and overloadin the channel. This ultimately results in a reduction of the quality ofthe signal output by the receiving terminal.

One type of communication network suitable for transmitting digitalinformation is the internet. Protocols which are used to carry voicesignals over an Internet Protocol network are commonly referred to asVoice over IP (VoIP). VoIP is the routing of voice conversations overthe Internet or through any other IP-based network.

Channels which route information via the internet are particularlysusceptible to degradation. Known methods for overcoming channeldegradation employ the use of encoding schemes that generate signalsthat are particularly robust to degradations in the channel. Howeverencoding a signal using such encoding schemes will remove some of theinformation from the signal. As such the quality of the received signalis reduced.

It is therefore an aim of the present invention to improve the perceivedquality of the received signal. It is a further aim of the presentinvention to provide a method of improving the quality of the receivedsignal without the use of complex computational methods.

SUMMARY

According to a first aspect of the present invention there is provided amethod of encoding a signal to be transmitted from a terminal via achannel in a communication network comprising; receiving the signal atthe terminal; determining characteristics of the channel; encoding afirst portion of the signal in accordance with a first encoding methodto produce a first encoded signal portion; encoding a second portion ofthe signal in accordance with a second encoding method to produce asecond encoded signal portion; wherein the first portion of the signalencoded in accordance with the first encoding method is dependent on thedetermined characteristics of the channel; and transmitting the firstencoded signal portion and the second encoded signal portion via thechannel.

According to a second aspect of the present invention there is provideda device arranged to encode a signal to be transmitted from a terminalvia a channel in a communication network comprising; a receiver arrangedto receive the signal at the terminal; a determiner arranged todetermine characteristics of the channel; a first encoder arranged toencode a first portion of the signal in accordance with a first encodingmethod to produce a first encoded signal portion; wherein the firstportion is dependent on the determined characteristics of the channel; asecond encoder arranged to encode a second portion of the signal inaccordance with a second encoding method to produce a second encodedsignal portion; and a transmitter arranged to transmit the first encodedsignal portion and the second encoded signal portion via the channel.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be carried into effect, embodiments of the present inventionwill now be described with reference to the following drawings:

FIG. 1 is a diagram of a communication system;

FIG. 2 is a graph showing the relative qualities of different encodingschemes;

FIG. 3 is diagram of a receiver and a transmitter according to anembodiment of the invention; and

FIG. 4 is a diagram of a receiver and a transmitter according to analternative embodiment of the invention.

DETAILED DESCRIPTION

Reference will first be made to FIG. 1, which shows a source terminal100 and a destination terminal 112 connected to a communication network104. The source terminal is arranged to transmit data to the destinationterminal 112 via the communication network 104. In one embodiment of theinvention the communications network is a VoIP network provided by theinternet. It should be appreciated that even though the exemplifyingcommunications system shown and described in more detail herein uses theterminology of a VoIP network, embodiments of the present invention canbe used in any other suitable communication system that facilitates thetransfer of data.

The terminals 100 and 112 may be, for example, a personal computer, agaming device, a personal digital assistant, a suitably enabled mobilephone, a television or other device able to connect to the network 104.

In order to overcome channel degradations in the network 104, it isknown to use parametric encoders which encode the parameters of a signaland transmit the parameters of the signal via the network. Theparameters are reconstructed at a decoder of the destination terminalaccording to a model. Parametric model based encoding methods areparticularly robust to degradations in the channel since a lost packetcontaining parameter information is relatively easily replaced.

Encoding schemes that are particularly robust to channel degradation,such as model based parametric encoders, limit the quality of theencoded signal. Even if the information transmitted in the signal isincreased by increasing the bit rate of the encoder, the decoded signalwill not reach transparent audio quality. The variation in signalquality with increasing bit rate for a model based encoder, isillustrated in FIG. 2. The signal quality for a waveform encoder whichdoes not use a model based encoding method is also illustrated. As shownin FIG. 2 model based coders are superior to waveform encoders at lowerbit rates, but do not converge towards transparent quality as the bitrate is increased.

The inventors of the present invention propose to compensate for thelimited quality of robust encoding methods by encoding part of thesignal with a high quality encoding method which does not use modelbased encoding methods.

According to an embodiment of the present invention a high quality nonmodel based encoder and a robust model based encoder are used to encodedifferent portions of a signal in dependence on the conditions of thechannel used to transmit the signal. Accordingly during poor channelconditions the signal may be encoded such that the quality of the signalis not substantially affected by degradations in the channel quality.However when the conditions of the channel are good the quality of thesignal may be enhanced by encoding part of the signal using a highquality non model based encoder.

The implementation of an embodiment of the present invention will now bedescribed with reference to FIG. 3.

FIG. 3 shows a signal transmitter 10 provided in the source terminal 100and a signal receiver 24 provided in the destination terminal 112. Itwill be readily appreciated that the transmitter 10 and receiver 24described may be present in both the source terminal and the destinationterminal.

As shown in FIG. 3, the signal transmitter 10 includes an analogue todigital converter 14, a core encoder 16, an enhancement encoder 20, achannel estimation unit 8 and a packetiser 22.

In operation, analogue signals such as speech are input into theanalogue to digital converter 14 from a microphone 12. The digitalsignals output from the analogue to digital converter 14 are input intoa core encoder 16.

According to an embodiment of the invention the core encoder uses amodel based parametric encoding method. In a preferred embodiment of theinvention the model based parametric encoding method provides a waveformapproximation of the input.

The proportion of the signal to be encoded by the core encoder isdependent on the conditions of the channel. In one embodiment of theinvention the conditions of the channel are estimated by the channelestimation unit 8. The channel estimation unit 8 may be arranged toestimate the available bandwidth of the channel. The channel estimationunit may determine the quality of the signal received at the destinationterminal by information provided by the destination terminal 112. Theinformation provided by the terminal 112 on the quality of the receivedsignal may relate to information on packet loss and variations in signalpropagation delay, otherwise referred to as jitter.

The enhancement encoder 20 is arranged to encode the portion of thesignal not encoded by the core encoder. According to an embodiment ofthe invention the enhancement encoder 20 is arranged to use a highquality non model based encoding method to encode the part of the signalnot encoded by the core encoder.

In a preferred embodiment of the invention the encoding scheme used bythe enhancement encoder is a waveform encoding scheme based on theModified Discrete Cosine Transform (MDCT).

The channel estimation unit is arranged to report at least some of thechannel parameters to the core encoder 16. Based on these parameters thecore encoder 16 decides what proportion of the signal input from theanalogue to digital converter 14 to encode.

In one embodiment of the invention the core encoder 16 applies analgorithm to the channel parameters to determine what proportion of thesignal to encode at the core encoder. In an alternative embodiment ofthe invention predefined ranges are set for a channel parameter, such asbandwidth, to determine which range the parameter falls within. The coreencoder is then arranged to encode a proportion of the signal whichcorresponds to the range to which the parameter belongs.

The signal input into the core encoder may be a frequency domain signal.In this case the portion of the signal encoded by the core and theenhancement encoder may be adjusted by adjusting the frequencies thatare encoded by the core encoder and the enhancement encoder.

According to one embodiment of the invention the core encoder may selecta cut off frequency f_(c) above which the core encoder will not encode.By reducing the cut off frequency the portion of information that iscontained in the encoded signal encoded by the core encoder is reduced.

The cut off frequency f_(c) may be determined from channelcharacteristics such as: the amount of packet loss and amount of jitteras reported by the terminal 112, and from the bandwidth available in thechannel. The inventors have found it particularly advantageous to setthe cut off frequency high, such that a larger portion of the signal isencoded using the core encoder, when the channel characteristicsindicate that the channel is of poor quality.

In one embodiment of the invention, the cut off frequency f_(c) isadjusted to be proportional to a channel degradation indicator value.The channel degradation indicator is calculated to be proportional to ameasure of packet loss and to a measure of jitter, and inverselyproportional to the available bandwidth.

In one embodiment of the invention, the cut off frequency f_(c) isproportional to the degree of packet loss and jitter only.

In one embodiment of the invention, the cut off frequency f_(c) isproportional to the degree of packet loss only.

In one embodiment of the invention, the cut off frequency f_(c) isproportional to the degree of jitter only.

In one embodiment of the invention, the cut off frequency f_(c) isinversely proportional to the available bandwidth only.

After the core encoder 16 has encoded a portion of the signal defined bythe cut of frequency the encoder is arranged to output the encodedsignal a(n) to the packetiser 22.

The enhancement encoder is arranged to encode the remaining portion ofthe signal. In a preferred embodiment of the invention the part of thesignal not encoded by the core encoder may be determined by subtractingthe information encoded by the core encoder from the signal output fromthe analogue to digital converter z(n) to generate a residual signalr(n). The information encoded by the encoder may be determined bypartially or entirely decoding the encoded signal a(n) to generate asignal x(n).

As shown in FIG. 3, the core encoder 16 is arranged to output the signalx(n) indicating the information that has been encoded by the coreencoder. The signal x(n) is provided to a mixer 18 where it issubtracted from the signal z(n) output from the analogue to digitalconverter 14.

In a preferred embodiment of the invention the core encoder 16 willencode the frequencies below the cut off frequency according to theparametric encoding method. The frequencies above the cut off frequencyf_(c) may be encoded by the encoder as a series of zeros. The encodedsignal is then either partially or entirely decoded to produce a signalx(n) which is subtracted from the signal z(n).

Encoding the signal at the encoder will remove some of the informationfrom the signal z(n) which is encoded. Thus the decoded signal x(n) willcontain less information than the signal z(n). As such when the signalx(n) is subtracted from the signal z(n) at the mixer 18 frequencies thatwere not encoded by the core encoder will be provided to the enhancementencoder in the residual signal r(n).

More specifically the residual signal r(n) will contain the originalvalues of signal z(n) that are above the cut off frequency f_(c) andadditionally any information that was not encoded by the core encoderbelow the cut off frequency f_(c), thereby accounting for the error ofthe core encoder.

The residual signal r(n) is encoded by the enhancement encoder using ahigh quality non model based encoding method. The signal y(n) encoded bythe enhancement encoder is then output to the packetiser 22.

The packetiser 22 is arranged to input the encoded signals a(n) and y(n)received from the core encoder 16 and the enhancement encoder 20respectively into the payloads of data packets. In one embodiment of theinvention encoded signal frames of the encoded signals a(n) and y(n) maybe transmitted in the same payload of a data packet. In an alternativeembodiment of the invention the separate data packets may be used totransmit frames of the encoded signals a(n) and y(n) respectively.Header information provided in the header of each data packet mayindicate the encoding scheme used for the frames contained in thepayload. The data packets may then be transmitted to the destinationterminal 112 via the network 104.

Reference will now be made to the signal receiving circuitry 24 providedin the destination terminal 112, which is also shown in FIG. 3. Thereceiving circuitry comprises a core decoder 30, an enhancement decoder28, a de-packetiser 26 and a digital to analog converter.

The data packets received by the terminal 112 are input into thede-packetiser 26 which is arranged to extract the encoded framesprovided in the payload. Frames that were encoded by the core encodermay be identified by the packetiser using specific information providedin the header of the packet, or from parameters that may be extractedfrom the encoded frames.

The de-packetiser 26 is arranged to output encoded frames that wereencoded by the core encoder to the core decoder 30. Similarly thede-packetiser is arranged to output encoded frames that were encoded bythe enhancement encoder 20 to the enhancement decoder 28.

The core decoder is arranged to decode the signal received from thepacketiser according to the encoding scheme used at the core encoder.The enhancement decoder is arranged to decode the signal received fromthe packetiser according to the encoding scheme used at the enhancementencoder.

The decoded signal output from the core decoder 30 is input into a mixer32. Similarly, the decoded signal output from the enhancement decoder 28is input to the mixer 32. At the mixer the decoded signals output fromthe core decoder and the enhancement decoder are combined. The output ofthe mixer is the reconstructed decoded digital signal.

The decoded digital signal output from the mixer 32 is then input intothe digital to analogue converter 34 which converts the digital signalinto an analogue signal. The analogue signal is then output to an outputdevice such as a loudspeaker 36.

In an alternative embodiment of the invention the signal z(n) may betransformed into the Linear Prediction (LP) domain by applying LinearPredictive Coding (LPC) to the signal z(n) before it is encoded. Thisembodiment of the invention will be described with reference to FIG. 4.

LPC is a known method of analyzing speech signals. LPC analyses speechsignals to produce a set of LP parameters and a LP residual signal. AnLP domain signal is less sensitive to packet loss because it is possibleto predict the parameters of a lost LP data packet.

FIG. 4 shows a schematic representation of a section of the transmittingcircuitry 10′ of the transmitting terminal 100 and a section of thereceiving circuitry 24+ of the receiving terminal 112. Components of thereceiving and transmitting circuitry that were described in relation toFIG. 3 are referred to using like reference numerals.

As shown in FIG. 4, the signal z(n) output from the analogue to digitalconverter is input into an LP analysis filtering and coding block 40.The LP analysis filtering and coding block 40 generates a set of LPparameters and an LP residual signal z*(n). The LP parameters are outputto the packetiser 22.

The LP residual signal z*(n) is input into the core encoder 16 and tothe mixer 18. The core encoder encodes information in the LP residualsignal z*(n) according to a model based encoding method that preferablyprovides a waveform approximation of the LP residual signal. The encodedsignal a*(n) is output from the core encoder to the packetiser 22. Thecore encoder also outputs a signal x*(n) representative of theinformation in the LP residual signal that was encoded by the coreencoder. The proportion of the information in the LP residual signalz*(n) that is encoded by the encoder may be controlled in dependence ofthe channel parameters as described previously.

The signal x*(n) is subtracted from the LP residual signal z*(n) at themixer 18. This produces a residual signal r*(n) that is input into theenhancement encoder 20.

The enhancement encoder is arranged to encode the residual signal r*(n)according to a high quality non model based encoding scheme. Theenhancement encoder outputs the encoded signal y*(n) to the packetiser22.

The LP parameters and encoded signals a*(n) and y*(n) are transmitted indata packets via the network to the destination terminal 112. In oneembodiment of the invention the LP parameters, and the signal frames ofthe encoded signals a*(n) and y*(n) may be combined in the payloads ofthe same packets. In an alternative embodiment of the invention the LPparameters, encoded signal frames of the signal a*(n) and encoded signalframes of the signal y*(n) may be transmitted in separate packets.

At the destination terminal the core decoder 30 is arranged to decodethe encoded signal a*(n) according to the encoding scheme used in thecore encoder 16. Similarly the enhancement decoder 28 is arranged todecode the encoded signal y*(n) according to the encoding scheme used inthe enhancement encoder 20.

The decoded signals output from the core decoder and the enhancementdecoder are combined to produce the LP residual signal. The LP residualsignal and the LP parameters are input to an LP synthesis block 41 whichis arranged to generate the digital signal z(n).

In an alternative embodiment of the invention the core encoder 16 onlyuses a waveform approximating encoding system when the signal includesvoiced sounds. Voiced sounds may be detected using a soundclassification method known in the art. When the sound is not classifiedas voiced the encoder may model the output as random white noise incombination with a spectral envelope.

In one embodiment of the invention the enhancement encoder may beswitched off, thereby not producing an enhanced encoded signal y(n). Theenhancement encoder may be switched off if it is determined that toomuch of the enhanced encoded signal, for example more than a thresholdamount, was not received by the encoder. The enhancement encoder may beswitched on again when channel conditions improve as detected by thechannel estimation block 8.

In one embodiment of the invention the proportion of the signal that isencoded by the core encoder 16 may be adjusted by adjusting the numberof bits available to the core encoder in relation to the number of bitsavailable to the enhancement encoder. For a fixed bit rate, reducing thenumber of bits available to the core encoder will increase the number ofbits available to the enhancement encoder, thus increasing theproportion of the signal encoded by the enhancement encoder.

In one embodiment of the invention the energy of the signal a(n) outputfrom the core encoder is scaled in order to compensate for theenhancement encoder being switched on and off. The energy of the signala(n) is multiplied by a scaling factor when the enhancement layer is offin order to maintain a correct signal energy.

In an embodiment of the invention the core encoder 16 uses an encodingscheme that does not employ inter-frame encoding methods. One example ofinterframe encoding is a method of encoding that determines thedifference between two frames and transmits only the information of thedifference between the frames. By using an encoding scheme that does notemploy interframe encoding methods, a packet may be lost withoutsignificantly affecting the perceived quality of the received signal.

In an embodiment of the invention described above the signal r(t) inputinto the enhancement encoder includes the portion of signal not encodedby the core encoder due to the encoding error of the core encoder, andthe portion of the signal falling outside the frequency range encoded bythe core encoder. In an alternative embodiment of the invention theenhancement encoder may be arranged to encode only portions of thesignal falling outside the frequency range encoded by the encoder. Thismay be achieved by explicitly dividing the frequency ranges of thesignals input into the core encoder and the enhancement encoder.Alternatively the residual signal may be filtered to remove thefrequencies encoded by the core encoder.

In one embodiment of the invention a set of cut off frequencies are usedto define the portion of the signal encoded by the core encoder and theportion of the signal encoded by the enhancement encoder. For exampletwo cut off frequencies may be used to define a range of frequencies tobe encoded by the encoder. This range may be varied in accordance withthe quality of the channel. If the channel quality is poor a smallerrange of frequencies may be encoded by the enhancement encoder.

Alternatively more than one range may be defined by a plurality of cutoff frequencies. These ranges may define frequency bands that are moreimportant to transmit reliably than others. For example harmonicfrequencies may be considered to be more important than otherfrequencies. According to this embodiment of the invention thecharacteristics of the signal may be analyzed to determined the harmonicfrequencies in the signal. The cut off frequencies may then be set suchthat more harmonic frequencies are encoded by the core encoder when thechannel degradation value is high. The enhancement encoder will thenencode the remaining frequencies.

According to an alternative embodiment of the invention, when more thanone frequency range is defined more than one enhancement encoder, orcore encoder may be used to encode different ranges of frequencies.

In one alternative embodiment of the invention, an adjustable filter maybe used to generate the residual signal instead of the mixer 18.According to this embodiment of the invention the core encoder is notarranged to output the signal x(n), instead the adjustable filter may bearranged to remove frequencies from the signal that are encoded by thecore encoder to produce the residual signal. In the case where the coreencoder encodes frequencies below a cut off frequency f_(c), theadjustable filter may remove the frequencies below f_(c) from the signalz(n) to produce the signal r(n) to be applied to the enhancementencoder.

While this invention has been particularly shown and described withreference to preferred embodiments, it will be understood to thoseskilled in the art that various changes in form and detail may be madewithout departing from the scope of the invention as defined by theclaims.

1. A method of encoding a signal to be transmitted from a terminal via achannel in a communication network comprising; receiving the signal atthe terminal; determining characteristics of the channel; encoding afirst portion of the signal in accordance with a first encoding methodto produce a first encoded signal portion; encoding a second portion ofthe signal in accordance with a second encoding method to produce asecond encoded signal portion; wherein the first portion of the signalencoded in accordance with the first encoding method is dependent on thedetermined characteristics of the channel; and transmitting the firstencoded signal portion and the second encoded signal portion via thechannel.
 2. A method as claimed in claim 1 wherein the second portion ofthe signal encoded in accordance with the second encoding method isdependent on the first portion of the signal.
 3. A method as claimed inclaim 1 wherein the second portion of the signal is a portion of thesignal that is not encoded with the first encoding method.
 4. A methodas claimed in claim 1 wherein the first encoding method is a model basedencoding method.
 5. A method as claimed in claim 1 wherein the secondencoding method is a non model based encoding method.
 6. A method asclaimed in claim 1 wherein the characteristics of the channel relate toat least one of channel capacity, channel bandwidth, channel jitter andchannel packet loss.
 7. A method as claimed in claim 3 wherein theportion of the signal that is not encoded with the first encoding methodis defined by at least one cut off frequency.
 8. A method as claimed inclaim 7 wherein the cut off frequency is adjustable.
 9. A method asclaimed in claim 8 wherein the cut off frequency is adjusted to beproportional to at least one of a measure of packet loss and a measureof jitter.
 10. A method as claimed in claim 8 wherein the cut offfrequency is adjusted to be inversely proportional to the availablebandwidth.
 11. A method as claimed in claim 8 wherein the cut offrequency is adjusted to be proportional to a channel degradationindicator value.
 12. A method as claimed in claim 11 wherein the channeldegradation indicator value is determined from at least one of a measureof packet loss, a measure of jitter and a measure of bandwidth of thechannel.
 13. A method as claimed in claim 7 wherein a plurality of cutoff frequencies defines at least one range to be encoded by the firstencoding method.
 14. A method as claimed in claim 13 wherein the atleast one range is adjusted in dependence on characteristics of thechannel.
 15. A method as claimed in claimed in claim 13 wherein the atleast one range corresponds to a harmonic frequency.
 16. A method asclaimed in claim 1 further comprising; decoding the first encoded signalportion to produce a first decoded signal portion; and subtracting thefirst decoded signal portion from the received signal to determine thesecond portion of the signal.
 17. A method as claimed in claim 1 whereinthe received signal is a Linear Prediction residual signal.
 18. A methodas claimed in claim 1 wherein the second portion of the signal is notencoded.
 19. A method as claimed in claim 1 further comprising detectingperiods of unvoiced sound in the received signal.
 20. A method asclaimed in claim 19 wherein periods of unvoiced sound are modeled bywhite random noise in combination with a spectral envelope by the firstencoding method.
 21. A method as claimed in claim 1 wherein the firstencoding method provides a waveform approximation of the receivedsignal.
 22. A method as claimed in claim 1 wherein the first encodingmethod is a non interframe encoding method.
 23. A method as claimed inclaim 1 wherein the second encoding method is a high quality encodingmethod.
 24. A method as claimed in claim 1 wherein the second encodingmethod is a waveform encoding method.
 25. A method as claimed in claim 1wherein the signal is and audio signal.
 26. A device for encoding asignal to be transmitted from a terminal via a channel in acommunication network comprising; means for receiving the signal at theterminal; means for determining characteristics of the channel; meansfor encoding a first portion of the signal in accordance with a firstencoding method to produce a first encoded signal portion; wherein thefirst portion of the signal is dependent on the determinedcharacteristics of the channel; means for encoding a second portion ofthe signal in accordance with a second encoding method to produce asecond encoded signal portion; and means for transmitting the firstencoded signal portion and the second encoded signal portion via thechannel.
 27. A device arranged to encode a signal to be transmitted froma terminal via a channel in a communication network comprising; areceiver arranged to receive the signal at the terminal; a determinerarranged to determine characteristics of the channel; a first encoderarranged to encode a first portion of the signal in accordance with afirst encoding method to produce a first encoded signal portion; whereinthe first portion is dependent on the determined characteristics of thechannel; a second encoder arranged to encode a second portion of thesignal in accordance with a second encoding method to produce a secondencoded signal portion; and a transmitter arranged to transmit the firstencoded signal portion and the second encoded signal portion via thechannel.